1. Field of the Invention
The present invention relates to methods for manufacturing micro-lenses of image sensors, and more particularly, to methods for manufacturing micro-lenses in array.
2. Description of the Prior Art
As the development of electronic products such as digital cameras and scanners progresses, the demand for image sensors increases accordingly. In general, image sensors in common usage nowadays are divided into two main categories: charge coupled device (CCD) sensors and CMOS image sensors (CIS). The use of micro-lenses significantly improves the photosensitivity of the image sensors by collecting light from a large light collecting area and focusing it onto a small photosensitive area such as a photodiode.
Please refer to FIGS. 1-3, which are schematic drawings illustrating a conventional method for manufacturing micro-lenses array. As shown in FIG. 1, a semiconductor substrate 10 including photosensitive regions 12 and image sensors such as CCD or CMOS image sensors is provided. Then a transparent dielectric layer 16, a color filter array (CFA) 18, and a transparent planarization layer 20 are sequentially formed on the semiconductor substrate 10. Each color filter unit of the CFA 18 corresponds to one photosensitive region 12. Then, a micro-lens material layer 30 composed of photoresist is formed on the planarization layer 20.
Please refer to FIGS. 2-3. Next, a conventional photolithography process is performed to pattern the micro-lens material layer 30 and to form an array 34 composed of a plurality of micro-lens blocks 32. Each micro-lens block 32 corresponds to one color filter unit and one photosensitive region 12. Then, a baking process is performed to melt and re-shape the micro-lens blocks 32 and to form micro-lenses 36 in array as shown in FIG. 3. Gaps 40 between each micro-lens 36 approximately are equal to widths of the image sensors 14.
Along with improvements in resolution and decreases in sizes of pixel and image sensor 14, requirements of zero-gap to the micro-lenses 36 emerges. However, according to the conventional method for manufacturing micro-lenses, the smaller gap 40 causes two adjacent micro-lenses 36 to overlap, and thus a bridge is formed between said two adjacent micro-lenses 36. In other words, the conventional method fails to provide zero-gap micro-lenses. Therefore, methods are provided by those skilled in the art to solve such problem. Please refer to FIG. 4, which is a set of checkerboard photomasks 50 and 52 having corresponding micro-lens patterns provided US. Patent Application Publication No. US 2003/0111770 A1. The checkerboard photomasks are provided to satisfy the requirement of zero-gap. Please refer to FIGS. 5-6. The checkerboard photomask 50 is used to pattern a micro-lens material layer (not shown) and to form a first set of micro-lens blocks 60. Then a first baking process is performed to form a first set of micro-lenses 62 as shown in FIG. 6. Please refer to FIGS. 7-8. Then the corresponding checkerboard photomask 52 is used to pattern another micro-lens material layer (not shown) and to form a second set of micro-lens blocks 64. Then a second baking process is performed to form a second set of micro-lenses 62 as shown in FIG. 8. Thus formation processes of a zero-gap micro-lenses array is completed.
However, the checkerboard photomasks 52 and 52 are high grade photomasks having slight and precise micro-lens patterns, therefore consideration of increasing cost and increasing possibility of failure in exposure, development, and etching processes is raised. More important, though the zero-gap micro-lenses array comprising the first set of micro-lenses 62 and the second set of micro-lenses 66 is obtained, influence on the first set of micro-lenses 62 during the second baking process cannot be completely prevented. Therefore bridge 68 still would be formed between the first set of micro-lenses 62 and the second set of micro-lenses 66 as shown in FIG. 8. In order to avoid said bridge, another approach is provided to form a zero-gap micro-lens array with three steps of forming the micro-lens material layer, three photolithography processes and three baking process with different temperatures. Such method is much more complicated, and still cannot completely prevent one set of micro-lenses from influence caused in other baking process which leads to forming the bridge. Therefore, a method that not only simplifies the whole process but also improves the result of formation of the micro-lenses is needed.